Welcome Dave. That's quite normal. You'll need to run Automatic Background Extractor (ABE)with the Target Image Correction set to subtraction, as a rule, with replace target image selected. If you don't want to view the Background image model, select that too.
The other tool, is Dynamic Background Extraction (DBE). It's use is more hands on/involved - the manual version of ABE and preferable in some cases.
Reset STF after the process completes ready for the next process.